1. INTRODUCTION

In human endeavour there's a fascination with the
"biggest and the best", or the "best and the brightest". It's a matter
for the social and psychological sciences to speculate on the reasons we
feel driven to give Oscars to the best movies and to climb the highest
mountains, but being human (well, most of them anyway), astrophysicists are
not immune to the desire to search for the Universe's own brand of the
biggest and the brightest. We tend to give them extreme names such as
ultra-this or hyper-that. In their rarity however, unusual
objects and environments teach us about the most extreme physical
processes in the Universe.

Which brings us to ULIRGs (or ULIGs to some) - Ultra Luminous InfraRed
Galaxies. There are also HLIRGs (or HiLIRGs or HyLIRGs, or indeed HyLIGs):
Hyper Luminous InfraRed Galaxies. These Oscar contenders have historically
been defined simply in terms of luminosity:
L8-1000µm between
1012 and 1013L for
the ULIRGs and > 1013L for
HLIRGs 1. What are these
dramatic objects, and why are
they among the brightest objects in the Universe? Does this also imply they
are the biggest in term of mass or size, or are they just superficial
fireworks that leave little lasting impression? What can understanding
these enigmatic objects teach us about the evolution of galaxies and how
the the Universe came to look as it now does?

The last few years have seen a dramatic shift in our perceptions of
ULIRGs. Once believed by many to be a rare curiosity - certainly
interesting, yes, but perhaps no more than a local oddity - they're
beginning to see center stage much more frequently. Ironically, this
increased interest in these rare objects has arisen because we now realize
that ULIRGs were once not nearly so rare as we find them to be in the local
Universe: pioneering submillimeter and millimeter surveys have demonstrated
that ULIRGs are many hundreds of times more numerous at z > 1 than
they are
locally. This in turn suggests that they played a much more important role
in galaxy formation and evolution than we imagined, and so understanding
them becomes of prime importance. Fortunately, this rise in the fortunes of
the ULIRG has occurred in an era when many new observing capabilities are
coming on line. Foremost among these is the Spitzer Space Telescope, with
its suite of deep imaging infrared cameras and its sensitive infrared
spectrograph. Spitzer's extensive first results on ULIRGs are just now
beginning to be published. Submillimeter and millimeter cameras are also
improving dramatically; upcoming facilities include AzTEC, SCUBA-2,
Herschel, and ALMA. We can also look forward to major new insights from
mid- and near-IR facilities such as ASTRO-F, WISE, and JWST, as well as
radio facilities such as the Square Kilometer Array, and new X-ray
facilities with high hard X-ray sensitivities such as Con-X and XEUS. The
timeliness of this subject is exemplified by recent reviews of The Cosmic
InfraRed Background
[Lagache, Puget &
Dole 2005],
Interacting Galaxies
[van Gorkum &
Hibbard 2005],
Megamasers
[Lo 2005],
Galactic Winds
[Veilleux, Cecil,
& Bland-Hawthorn 2005],
and High Redshift Molecular Gas
[Vanden Bout
& Solomon 2005],
all of relevance to ULIRGs.

ULIRGs were first discovered in large numbers by the Infrared Astronomical
Satellite in 1983, and were found to be comparatively rare locally, with a
space density several orders of magnitude lower than that of normal
galaxies, and possibly a factor of a few higher than QSOs. Followup
observations show that most, if not all ULIRGs are found in major disk
mergers, and that the central few hundred pc of their nuclear regions
harbour very large masses of gas and dust. The power source behind the IR
emission is some combination of a large population of hot young stars (a
`starburst' 2) or a
very massive black hole accreting matter at a rapid rate (which for the
remainder of this review we refer to as an `AGN'). Though distinguishing
between the two initially (and even now) proved to be very difficult, it is
now thought that, at least locally, ULIRGs are mainly powered by a
starburst, but frequently with a significant AGN contribution. Local ULIRGs
reside in relatively low-density environments (not
unexpectedly, since relative velocities are thought to be too high for
mergers to occur in rich, virialized environments), and are expected to
evolve into spheroidal systems as the galaxy mergers that appear to trigger
ULIRG activity progresses.

Even IRAS was sensitive enough to determine that there has been very strong
evolution in the ULIRG (and LIRG) population with redshift out to at least
z ~ 0.5, with approximate form (1 + z)4. IRAS
also found ULIRGs out to
extremely high redshifts, including the famous, lensed, F10214+4724 at
z = 2.286. This strong evolution was confirmed with results from the
Infrared Space Observatory which, although covering much smaller areas than
IRAS, could probe this evolution out to z ~ 1 due to its greater
sensitivity (Figure 1). This evolution was later
recast as the now
ubiquitous `star formation history of the Universe' figures, which show
that LIRGs rather than ULIRGs are responsible for the bulk of the evolution
seen since z ~ 1 in IR galaxies. ULIRGs, however, did not slink into the
shadows; on the contrary they returned triumphant with the advent of
submillimeter imaging surveys, which came shortly after ISO and can in
principle probe IR-luminous galaxies up to z ~ 7. These sub-mm
surveys showed that ULIRGs are orders of magnitude more numerous at
z > 1 than
locally, outnumbering optically bright QSOs at those redshifts by a large
margin. Followup observations showed that these distant ULIRGs bear many
similarities to their local cousins, but also exhibit some key differences,
and that they may signpost the obscured phases of the very dramatic events
suspected of building the most massive galaxies seen in the local Universe.

When considered within the framework of modern theories for the formation
of galaxies and large-scale structure, it seems initially surprising that
there are many more ULIRGs at high redshift than locally, because in early
implementations of the `hierarchical buildup' paradigm, large galaxies
build up slowly from the mergers of smaller systems. This is in contrast
to the early `monolithic collapse' models
[Eggen, Lynden-Bell
& Sandage 1962],
where ellipticals
formed early in a dramatic burst of star formation, which had been largely
supplanted in favour of hierarchical models. The discovery of so many
ULIRGs at high redshifts caused hierarchical models of the time major
difficulties in making enough distant systems with such high star formation
rates. The basic dark matter halo growth theory, described by an
extended/modified Press-Schechter formalism, does however allow for rapid
baryon accumulation in very massive dark matter halos, and recent galaxy
formation models are having greater success in producing the observed
number of ULIRGs in sub-mm surveys, albeit with some stringent assumptions.

In this review, we will therefore focus on selected key topics: (1) our
understanding of the astrophysics of local ULIRGs, and in particular the
relative importance of star formation versus AGN in powering ULIRGs, (2)
similarities and differences between local and high-redshift ULIRGs, and
(3) the relationship between ULIRGs and the formation of large-scale
structure and of galaxies as a function of redshift. Since the study of
ULIRGs clearly connects to many major disciplines of observational and
theoretical extragalactic astronomy we cannot hope to cover all topics of
relevance to them in this review. Nor can we completely review all recent
published studies of ULIRGs; excellent ULIRG papers simply abound. We
therefore highlight the most recent advances and our perspective on the
most important questions concerning their study within the framework of
galaxy and structure formation.

In section 2 we provide brief historical
context to the discovery of ULIRGs and their evolutionary role. In
section 3 we review current understanding
of the astrophysics of local ULIRGs by wavelength, and in
section 4 we summarize
local studies into a picture of ULIRG nature and evolution in the local
Universe. In section 5 we review
observations of ULIRGs at
higher redshifts, based primarily on data from ISO, SCUBA, HST and
Spitzer. Section 6 places these studies
into the context of
structure formation and reviews their role within galaxy formation
scenarios. Finally, section 7 highlights
the key open questions and our perspectives of where the answers are
likely to come from.

For the remainder of this review we usually refer to ULIRGs and HLIRGs
combined as ULIRGs, because many of the earlier works used the term
"ULIRG" to refer to all objects above 1012 in
L and the
term HLIRG has been in only recent and inconsistent use. We assume
H0 = 70 km s-1 Mpc-1,
= 1, and
=
0.7. Luminosities are quoted in units of bolometric
solar luminosities, where
L =
3.826 × 1026 Watts. Unless
otherwise stated, the term `IR' or `infrared' luminosity refers to the
integrated rest-frame luminosity over 1-1000 or 8-1000 µm
(which differ very little for most SEDs).

1 The supporting cast consists of LIRGs
(or LIGs), the much more common lower luminosity
understudies of the prima donnas, with L8-1000µm
between 1011 and 1012LBack.

2 for our purposes defined as a star
forming event with a gas exhaustion timescale very short compared to the
Hubble time
Back.